1. Field of the Invention
The present invention related to a packaging structure of LED and more particularly, to a package structure of overlap bonding the LEDs in cascaded by chip direct bounding with transparent conductive layer and reflected metal layer, and capable of generating a full color light or white light by mixing lights emitted from the overlap LEDs.
2. Description of Relative Prior Art
As a good light source and device made by semiconductor material, LEDs possesses advantages of small size, long life-time, low driving voltage, rapid response, and good oscillation-proof, etc.
By changing the semiconductor materials and device structures, LEDs with different visible and invisible colors can be designed, wherein AlGaAs, InGaAlP and InGaN are suitable for producing LEDs with high luminance over 1000 mcd.
When producing red or infrared LEDs with high luminance by AlGaAs, an LPE process and DE structure devices are used for industrial mass production.
InGaAlP can be used to produce red, orange, yellow and yellow-green LEDs, and an MOVPE (Metal Organic Vapor Physical Epitaxy) process, double hetero (DH) junction structures, and quantum well (QW) structures are provided in efficient mass production. FIG. 1 shows the cross section of a traditional InGaAlP/GaAs or InGaAlP/GaP yellow semiconduct;or LEDs 10, wherein an InGaAlP epitaxial layer 14 is formed on an n-type GaAs substrate 13. A positive bond pad 11 is formed by gold (Au) for being connected to an anode package leg, and a negative bond pad 12 is formed by Al or Au and connected to a cathode package leg.
InGaN is suitable for producing green, blue and ultra-violet LEDs with high luminance by high temperature MOVPE processes, wherein DH structures and QW structures are used, too. FIG. 2 shows the cross section of a traditional blue LED die 20, wherein a substrate 23 is formed by transparent sapphire. An upper p-type InGaN epitaxial layer 25 and a lower n-type InGaN epitaxial layer 24 are deposited on the substrate 23. A positive bond pad 21 is formed on the p-type InGaN for being connected to an anode package leg, and a negative bond pad 22 is formed on the n-type InGaN for being connected to a cathode package leg. Alternatively, the n-type InGaN epitaxial layer 25 can be epitaxied on the p-type InGaN epitaxial layer 24. As shown in FIG. 2, the sapphire substrate 23 as a support base results in a different connecting type for the negative bond pad 22 from FIG. 2.
In order to get colors other than red, green and blue light by adjusting the intensity of the primary color of red, green and blue lights as shown in the chromaticity diagram in FIG. 3. In the figure, if we adjust the blue and yellow light intensity along line AB, we shall have a white light in the intersection of line AB and CD; apart from the intersection and near point A we have bluish color. If there are red, blue and green primary colors, by adjusting the intensity of each color. We shall have full color light source. As shown in FIG. 4, a conventional method of forming a full color LED, a red, a green and a blue primary color LEDs are packaged with chips 401, 402 and 403 in a row or in array by die bond on a PC board. The power of the red LED is apply to the positive pad 406 and via the ground pad 404 on the PC board 405 to the negative of the source, the same method is apply to the green and blue LEDs. Generally, a constant current source of 20 mA is used as the power source as shown in FIGS. 4 and 5.
For example, 20 mA with voltage of 2V is applied to the red LED 401; 20 mA with voltage of 3.5V is applied to the green LED 402 and 20 mA with voltage of 3.5V is applied to the blue LED 403, a white light is obtained and the power consumption is 180 mW (20xc3x972+20xc3x973.5+20xc3x973.5=180 mW ); if full color is needed, the current is still kept at 20 mA, but by control the lighting period of each LED to combined the light to obtain different colors. The control accuracy is difficult to obtain, and the design of the power control IC is complicated, more ever, for constant current much heat generated and heat dispersion is not easy, it causes short lifetime of the LED. In near field observation, three colors is still observed, full color can be observed only for far field observation, so it does not satisfy the need of the market.
The object of the present invention is to provide an overlap cascaded packaging structure of a full color semiconductor LED to satisfy near field or far field display application.
Another object of the present invention is to provide an overlap cascaded packaging structure of a full color semiconductor LED with reflected metal layer to enhance the intensity of light.
Another object of the present invention is to provide a light source with metal oxide transparent conductive layer to die bond multi-chip to form overlap cascaded packaging structure. The structure has good resolution and is satisfied for both near field or far field observation.
The other object of the present invention is to provide an overlap cascaded packaging structure of a full color semiconductor LED to obtain light source of different colors by controlling the magnitude of the current to control the intensity of the lights, this can achieve less power consumption and increase heat dispersion efficiency.
In order to achieve the above objects, a full color semiconductor LED, by using metal reflected layer and transparent conductive layer deposited on the red, blue, green or yellow LED to overlap bond the chips in cascade on a pc-board, at least includes: (a) a pc-board with a reflected metal layer deposited on it, the metal form a pattern which included a chip bonding pad, a red light positive bond pad, a blue light positive bond pad, a green light positive bond pad and a common negative bond pad; (b) a first red LED chip with a transparent conductive layer of red light positive electrode on the top side of the chip and a reflective metal layer of red light negative electrode on the bottom side of the chip, on one side of the positive electrode, a square reflective metal layer is deposited on the positive electrode to form a red light positive bond pad and to reflect the red light, the red light LED chip is bonded directly on the chip bonding pad of the pc-board; (c) a second blue LED chip with a transparent conductive layer on both side of the chip, the top layer form a positive electrode of the blue LED, on one side of the positive electrode, a square reflective metal layer is deposited on the positive electrode to form a blue light positive bond pad and to reflect the red light and blue light, on one side of the chip, a strip of the p-type semiconductor is etched away and a reflected metal layer is deposited to form a blue light negative bond pad, the blue light LED chip is overlap bond directly on the red LED chip in cascade; (d) a third green LED chip with a transparent conductive layer on both side of the chip, the top layer form a positive electrode of the green LED, on one side of the positive electrode, a square reflective metal layer is deposited on the positive electrode to form a green light positive bond pad and to reflect the red light, the blue light and the green light, on one side of the chip, a square of the p-type semiconductor is etched away and a reflected metal layer is deposited to form a green light negative bond pad, the green LED chip is overlap bond directly on the blue LED chip in cascade; (e) using metal wire to connect the positive bond pad and negative bond pad of the first red LED, the second blue LED and the third green LED to the positive bond pads and negative bond pad of the pc-board.
The reflective metal layer on the pc-board and on the chip is aluminum, copper or gold, the thickness of the reflective metal layer isl 1000 xc3x85 to 20000 xc3x85. It is better to be 2000 xc3x85 to 5000 xc3x85.
The red LED chip is form by epitaxy n-type and p-type InGaN on a transparent sapphire substrate, the chip size is about 300 xcexcm to 900 xcexcm rectangular.
The green LED chip is also form by epitaxy n-type and p-type InGaN on a transparent sapphire substrate. The transparent conductive layer is Indium Oxide (In2O3)) or other transparent conductive layer, the thickness of the transparent layer is about 200 xc3x85 to 10000 xc3x85, the better is 500 xc3x85 to 1000 xc3x85. The size of the positive bond pad on the red, blue and green LED is about 50 xcexcm to 200 xcexcm, the better is 100 xcexcm. The die is bonded at a temperature of 450xc2x0 C. to 550xc2x0 C., the eutectic point of the metal or metal oxide, or bonded by ultrasonic bonding, or adhered by a transparent epoxy material.
Another overlap cascaded packaging structure which is suitable for far field observation is form by overlap bonding of a blue light LED on a green light LED and a green light LED on a red light LED.